1. Field of the Invention
The present invention relates to a bicycle rim and a wheel having such a rim.
2. Description of Background and Relevant Information
In a known manner, a bicycle wheel includes a central hub, a peripheral rim and connecting spokes between the rim and the hub. The spokes are distributed in two sets, and their number can vary from 6 to 18, or even more, in each of the sets, depending on the degree of stiffness desired for the wheel.
The rim is made from a bent profile element that has the shape of a casing, i.e., that is box-shaped in cross-section, with an upper bridge, a lower bridge, and two side connecting walls. For certain profiles, the lower bridge is sometimes very narrow and merges with the side walls. For certain rims, the two bridges are joined by one or several intermediate ribs.
There are two main construction methods of connecting the spokes to the rims. Either the means for retaining the spoke is supported on the upper bridge of the rim; in this case, it is generally double eyelets that retain the spoke nipple and take support on both the upper bridge and lower bridge of the rim, or the spoke is only connected to the lower portion of the casing, i.e., most often to the lower bridge with a single eyelet, or even without any eyelet. For certain rims having a tapered profile, the spokes may be connected to the side walls of the rim.
It can be interesting to connect the spokes only in the lower portion of the casing, especially in order to obtain a tubeless wheel. To this end, blind holes that do not extend through the upper bridge can be made in the lower bridge, for example. Such a rim is disclosed in application WO93/09963, or patent application EP 818 328, more particularly the embodiment of FIG. 13. In both cases, the spokes are screwed by means of an end-piece in the threaded holes of the lower bridge of the rim. Since the upper bridge does not have any hole, except that provided for the valve, such a rim is perfectly adapted for a tubeless wheel.
The invention relates more particularly to this category of rims.
The important properties of a rim are its static strength, its dynamic fatigue strength and its weight, which determines its inertia.
For a wheel, the important properties are the weight, the inertia, its strength and stiffness under front and lateral stress.
Recently, it has been sought to lighten the wheels by reducing the rim material mass in order to also reduce its inertia and the inertia of the wheel.
In this field, a first constraint is the wall minimum thickness that is imposed by the operation of drawing or extruding the profile element of the rim. Reasonably, it is indeed not possible to reduce this thickness below 0.8 millimeters.
However, certain rim profiles with an types of alloys can support wall thicknesses that are lower than the minimum thickness imposed by the drawing operation. Therefore, one has begun reducing the rim wall thickness after drawing, by chemical machining, for example, as is described in patent application BP 715 001, or EP 714 792.
With respect to connecting the spokes, these two documents recommend the use of double eyelets. These rims therefore belong to the first of the aforementioned categories. The double eyelets yield good results; however, their correct positioning is difficult to control due to the manufacturing tolerance of the section of the rim, which influences the distance between the two bridges, and to the manufacturing tolerance of the eyelet itself. The inflation can also influence the tightening which the eyelet axerts between the two bridges or the distribution of the forces between these two bridges. An incorrectly positioned, i.e., overly tightened, double eyelet can produce less satisfactory results than a single eyelet.
The reduction in the wall thickness essentially affects the lower bridge of the rim, for it is not subject to the stresses coming from the pressure in the tube, as well as to the stresses due to the impacts of the wheel. Therefore, it is logical to connect the spokes to the upper bridge of the rim in order to relieve the lower bridge of the stresses generated by the tension of the spokes.
The construction for the second category of rims to which the invention relates is different. Indeed, the tension of the spokes in this case is mainly sustained by the lower portion of the casing. The thickness of the lower bridge cannot be reduced in a uniform manner, as disclosed in EP 715 001, for example, without affecting the fatigue strength of the bridge. This document does provide the use of shields positioned in the vicinity of the holes so as to locally reinforce the thickness in these zones after the chemical attack. However, such a technique proves cumbersome to implement. Moreover, the reduction in thickness obtained is sharp, which does not resolve the problem of fatigue strength in a satisfactory manner.
An object of the invention is to propose a rim with a connection of the spokes in the lower portion of the casing, which has improved properties, i.e., whose weight is reduced by a machining of the wall, and whose fatigue strength is not significantly affected by the decrease in weight.
Another object of the invention is to provide a rim having an improved appearance, i.e, whose machined surfaces obtained in view of the reduction in weight form a support that makes it possible to produce a remarkable-aesthetic effect.
Yet another object of the invention is to provide a bicycle wheel whose spokes are connected to the lower portion of the casing and which has improved weight and inertia properties.
Other objects and advantages of the invention will become apparent from the description that follows.
The bicycle rim according to the invention has an annular shaped, or profile, element whose cross-section includes a casing demarcated by an upper bridge and a lower portion having a lower bridge and side walls, the lower bridge or the side walls having holes distributed evenly and provided for connecting the spokes, the holes and their immediate vicinity defining boring zones.
The thickness of at least one of the walls of the rim constituted by the lower bridge and the side walls has a nominal value in the boring zones and a lower value than the nominal value in the intermediate zones located between two boring zones, and in that the thickness of this wall varies progressively from the nominal value to the lower value to the nominal value.
The invention is based on the following finding. For a connection of the spokes to the lower bridge of the rim, the stresses in the lower bridge are substantial in the boring zones, but decrease rapidly as soon as one moves away from these zones. Thus, the wall thickness can be reduced between the different boring zones without affecting the fatigue strength of the rim significantly. The progressive variation in thickness furthermore ensures a homogenous distribution and scattering of these stresses.